1. Field of the Invention
The present invention relates to a single-ended input to differential output low noise amplifier with a cascode topology, and more particularly to an architecture and concept with a cascode topology.
2. Description of Related Art
FIG. 1 shows a function block diagram of a front-end receiver of the prior RF system. An LNA 14 is one part of the receiver in the communication system. The LNA 14 amplifies a received signal and dominates the noise figure of the receiver. After an antenna 11 receives a high frequency signal, the signal is transferred to a duplexer 12, filtering with a first filter 131, amplifying with a low noise amplifier (LNA) 14 and filtering with a second filter 132, subsequently mixing with a mixer 15 and ongoing continuing the procedure of the signal processing.
Reference is made to FIG. 1. In general, the LNA 14 is a type of single-ended input to single-ended output. Accordingly, the mixer 15 located behind the LNA 14 must be a type of single input, so a common mode noise of the receiver and an output signal that feeds through from an oscillator 16 to the mixer 15 do not effectively decrease. Nonetheless, a LNA with differential output can resolve the above problems.
A differential pair with differential input and output is usually used in the differential output LNAs. A balun must be added in front of the differential input to differential output amplifier, for converting an output signal of the antenna 11 to a differential output signal. This matter has some drawbacks. The balun is an added cost, and the noise figure of the whole front-end receiver is therefore increased by the balun.
FIG. 2 shows a circuit diagram of a single-ended input to differential output LNA with a passive transformer. A transformer T1 is made of wire-wrap of IC metal. The T1 is electrically connected a first transistor M1 and a second transistor M2 of a differential pair for converting single-ended signal to differential signal. At the same time, FIG. 2 has a DC current source IS and an output matching impedance ZL1 and ZL2. This matter has some drawbacks. The transformer takes up die area, the current of IS must be the sum of the currents of two transistors, and the noise figure of the LNA is increased.
FIG. 3 shows a circuit diagram of a single-ended input to a differential output LNA under one terminal connected the ground. Although cost and area of the transformer T1 are reduced, the complexity of design of the LNA increases when the common mode point is unbalanced for operating at high frequencies. Likewise, the current of IS still is the sum of the currents of two transistors.